There are widely known self-centering triple-jaw chucks for securing workpieces to be machined in lathes and boring machines, comprising a central driven rod actuating through a system of effort-transmitting members three radially movable jaws uniformly spaced in the housing of the chuck.
The use of such chucks, particularly when workpieces are machined in multi-station machine tools with a considerable cutting effort, more often than not results in considerable elastic deformation of the workpiece being machined, such deformation being capable of distorting beyond the permissible tolerance the shape of the surfaces being machined, particularly when the workpiece is inadequately rigid. This distortion of the shape is the more significant, the greater the requirements are concerning the precision of the machining of the surfaces, the abovementioned elastic deformation also influences the precision of the working of articles which can be considered rigid.
To reduce this distortion of the shapes of the surfaces being machined, in other known devices each jaw of a triple-jaw chuck is provided with a rockable clamping member, which enables the chuck to secure a workpiece by the efforts of the three pairs of clamping elements, the elements of each pair extending parallel with each other. However, in this case the clamping elements of the jaws cut into the workpiece since the elements do not move strictly toward the axis of the chuck, so that the chuck is practically incapable of centering and securing the workpiece with the required accuracy. Furthermore, with the jaws provided with rockable clamping elements, the rigidity of the chuck is somewhat affected, the same as the workpiece-holding capacity thereof. Besides, the torque created by the cutting effort might result in the rearrangement of the clamping forces acting upon the workpiece, and, hence, in misbalance of these forces.
Thus, the abovedescribed known chucks do not ensure reliable centering of workpieces, do not eliminate elastic deformation, to say nothing of their workpiece-holding capacity being impaired, which affects the overall productivity of the machining process. To attain the required high accuracy of the machining of a workpiece, it becomes necessary to introduce additional operations, either pre-machining or finishing operations with the resultant decreased productivity of the machining process. Costs and the amount of labour consumed are significantly increased.
Recently, in order to improve the accuracy of turning operations, attempts have been made to create self-centering chucks for securing workpieces by their unmachined base or reference surfaces, with the use of six radially reciprocable jaws. These last-mentioned chucks have two groups of jaws uniformly spaced about the circumference of the chuck, the two groups being actuated by two separate drives, viz. one group of three jaws is actuated by a mechanical drive, while the other group is actuated manually. In other cases both groups are actuated by a manual drive of a successive action; in still other cases both groups of the jaws are actuated by two independed mechanical drives.
The swiftness of the action of the last-mentioned clamping chucks is inadequate; this means that additional time is required to position and secure a workpiece; besides, these chucks fail to ensure accurately timed application of the clamping forces by all the jaws, which would not provide for complete elimination of elastic deformation of workpieces which are not adequately rigid. Moreover, the last-described chucks have a complicated and bulky structure, which practically precludes their employment in several types of machine tools, e.g. in automatically operated ones.